Generally, polymer injection molding machines include an elongate barrel with a bore extending therethrough. A powered elongate screw is housed within the bore of the barrel. Rotation of the screw within the bore plasticizes and melts polymer pellets fed into the bore from a hopper. In an injection stroke, forward translation of the screw relative to the bore forces a shot of accumulated molten polymer from the bore for injection into a mold.
In some instances, a filter assembly is positioned at the end of the barrel for filtering contaminants from the molten polymer as the polymer is forced in a shot from the bore by the forward translation of the screw. The filter assembly can include a mechanism which removes contaminated filter elements from the polymer flow path and then repositions clean filter elements back into the flow path. A drawback of such a filter assembly is that filtering the molten polymer during the injection stroke of the screw results in a very high flow rate of polymer through the filter assembly which increases resistance to the injection stroke as well as the time required to perform the injection stroke. In addition, the filter elements usually require handling by the machine operator, which, depending upon the design of the filter assembly, can be unwieldly. Previous attempts to position a purgable filter within the barrel to filter polymer prior to the injection stroke in order to avoid these problems require special channels formed within the barrel for purging contaminants from the filter. Such channels can be difficult to manufacture.